I have a Solid State Relay (https://www.sparkfun.com/products/10636) in a project switching 120V AC. An Atmega328 digital pin is connected through a 200ohm resistor to the SSR +, and the SSR - is connected to ground. The one pin on the load side of the SSR is connected to 120V AC to test it.

My problem is that when the SSR should be off, about 12V AC (I think it's AC) leaks through. When its on, the 120V AC switches on like it's supposed to.

I'm rather new to controlling 120V, and I dont know if the leakage is normal or did I break the SSR? Any help is appreciated. Thanks.

Yes, that's normal. It's not a fully open or closed switch. It, its triac, is a semiconductor - so when it's off it's a large resistance, not an infinite resistance. Does that make sense?

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Okay...that didn't seem to work. Now across the relay contacts there is 120V when the connection is supposed to be open, I assume this is the voltage flowing through the resistor. I just want the relay to be like a light switch: either on or off. But 0.1v would be okay I think..?

Is there any real advantage to using solid state relays or am I just better off with a mechanical one?

Solid State relays have leakage. If you want to repeatedly switch something on / off, use them.But when you want the SSR to be fully off, say after pressing an off switch, a mechanical relay should be across the load to take it off the SSR.

If you aren't switching repeatedly, then use a mechanical relay.In lights, I would use an SSR with a mechanical relay to drive a strobe effect on a light.I would use a mechanical relay itself to drive the lights on at night, off in the morning.

Well I do only need to switch the relay on an off once per day, so maybe the SSR isn't needed. I can't draw the schematic now but I can try to describe it.

The SSR control is attached to the atmega328 through a 200ohm resistor. I believe that part is correct. Then one of the load pins is attached to 120V AC , and the other will go to the load. I am currently measuring the switched load pin against the other leg of the 120V. Am I measuring it wrong? I can post a real schematic soon.

The triac in an SSR leaks very little, however there is additional circuitry for triggering the SSR that draws a small amount of current, and this is why you can detect some voltage when you connect a high resistance load such as a meter. You can find the leakage current on the datasheet for the SSR, but 2 or 3mA is typical. For typical high-current loads, the leakage is not a problem. If you use the SSR to drive a high power 240V 3-phase relay, then I suspect that this small amount of current will not be a problem. You can confirm that by checking how much current the big relay coil takes, which will be specified on its datasheet.

btw you almost certainly don't need the 200 ohm series resistor, because this is normally built into the SSR. If the input is specified as e.g. "3 to 20V" then it has a series resistor. If it didn't include a series resistor, then it would specify an operating current range, along with a typical forward voltage, just like a LED.

EDIT: That Sharp SSR has an unusually low off-state current, the datasheet says 100uA maximum. You can safely ignore it for the purposes of driving another relay. And (unusually for an SSR) it DOES need the series resistor.

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